Casey M. Holliday

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Vertebrate Functional Morphology & Evolution

 

New Insights into Dinosaur Jaw Muscle Anatomy

C. M. Holliday. 2009. The Anatomical Record 292:1246-1265.

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Dinosaurs evolved an extraordinary diversity of head shapes and feeding behaviors from bone crushing tyrannosaurs to the grinding machines that are hadrosaurs, to the stripping and chopping mouths of sauropods and ceratopsians. Besides these well-known charismatic animals, numerous other taxa have evolved bizarre heads that can befuddle paleontologists and comparative anatomists looking to reconstruct the form, function, and behavior of dinosaurs. This paper is intended to serve as an anatomical atlas to those that want to flesh out their animals, to test feeding behaviors using various imaging and modeling methods, and simply to envision the jaw muscles, trigeminal nerves, blood vessels and other soft tissues that occupy a significant portion of the head. Hopefully, this paper, among others, will add to the anatomical foundation that researchers use to bring their dinosaurs back to life.

The skull material of numerous specimens were analyzed using 1st hand and 3D imaging techniques to identify where jaw muscles attached. These inferences were based on data from extant taxa such as lizards, crocodylians, and birds, as well as numerous fossil taxa including basal archosaurs and crocodyliforms. These complementary works can be found at Holliday and Witmer 2007, Archosaur Adductor Chamber Topology (J Morph) and Holliday and Witmer in press-soon, Crocodyliform Braincase Evolution (JVP).

Not surprisingly, inferring where muscles attached can be challenging given the vagaries of direct vs tendinous attachments, positional shifts that  occur during the evolution of particular clades, and sometimes a lack of transitional forms. Regardless, we can be fairly certain where many muscles attached. These muscles, along with consistent patterns of nerve and artery topology, give us an informed view of what the temporal region of the dinosaur head looked like. 

 

 

Funded by: National Science Foundation: IBN-0407735; Marshall University School of Medicine, The Jurassic Foundation, Ohio University Student Enhancement Award

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Fig 1. Anatomy and phylogeny of dinosaurs and their extant archosaur and outgroup relatives. A, Phylogenetic relationships of dinosaurs, extant bracketing taxa, and outgroups. Adapted from Gauthier, 1986, Sereno, 1997, Senter, 2007, Butler et al. 2008. Extant clades in bold. Numbered nodes: 1, Sauropsida; 2, Archosauria; 3, Dinosauria; 4, Ornithischia; 5, Thyreophroa; 6, Ornithopoda; 7, Ceratopsia; 8, Saurischia; 9, Sauropoda; 10, Theropoda; 11, Coelurosauria; 12, Maniraptora; 13, Aves. B, Muscle attachment surfaces in the skull and mandible of the monitor lizard (Varanus exanthematicus); C, Muscle attachment surfaces in the skull and mandible of the chicken (Gallus gallus); D, Muscle attachment surfaces in the skull and mandible of the alligator (Alligator mississippiensis. Homologous muscles are similarly color coded throughout figures. Fig. 2. Osteological correlates of the orbitotemporal and temporal regions. Photos have been standardized to left, lateral views for simplicity. White arrows: correlate for m. levator pterygoideus; black arrows, correlate for m. protractor pterygoideus. Scale bar equals 1cm. A, Brachylophosaurus (juvenile; MOR 1071); B, Brachylophosaurus (adult; MOR 1071); C, Lambeosaurus (CMN 2869); D, Protoceratops (IGM 100/1246); E, Pachyrhinosaurus (TMP 89.55.188); F, Triceratops (MOR 1120); G, Plateosaurus (AMNH 6810); H, Camarasaurus, CM 11378; I, Nigersaurus, MNN GAD 512; J, Majungasaurus (FMNH PR2100); K, Daspletosaurus (TMP 94.143.1); L, Tsaagan, IGM 100/1015. Fig. 3. Osteological correlates of the temporal region and palate. Scale bar equals 1cm. Braincases in dorsal view: A, Edmontosaurus (CMN 2289); B, Leptoceratops (CMN 889); C, Troodon (TMP 82.19.23); D Carcharodontosaurus (SGM Din 1; left, lateral view); E, Herrarasaurus (MCZ 7064); F, Daspletosaurus (CMN 8506). Palates: G, Triceratops (MOR 699; left, lateral view) H, Same as G, close up of left, medial view; I, Brachylophosaurus (MOR 1071; left, lateral view); J, Plateosaurus (AMNH 6810, left, lateral view); K, Camarasaurus (CMN 11378 left, lateral view of CT-scan-rendered image); L, Daspletosaurus (TMP 2001.36.1; left, lateral view).
Fig. 4. Osteological correlates of the mandible. Scale bar equals 1cm. A, Thescelosaurus (ROM 3587; left, lateral view); B, Edmontosaurus (CMN 2289; right, medial view); C, indet. hadrosaur (ROM 1949; left, lateral view); D, Leptoceratops (CMN 8889; left, lateral view); E, Centrosaurus (ROM 767; left, lateral view); F, Stegosaurus (CM 41681; right, medial view); G, Panoplosaurus (ROM 1215; right, medial view); H, Plateosaurus (AMNH 6810; right, medial view); I, Camarasaurus (CM 11378; right, medial view); J, Dromaeosaurus (AMNH 5356; left, lateral view); K, Tyrannosaurus (MOR 008, right, medial view); L, Caenognathus (ROM 8776; left, lateral view). Fig. 5. Muscle attachments, trigeminal nerves, and arteries of the braincases of select dinosaurs in left, lateral view. A, Triceratops; B, Brachylophosaurus; C, Tyrannosaurus. Fig. 6. Palate anatomy and evolution in non-avian dinosaurs. A, Palate morphology and homoplastic epipterygoid loss in non-avian dinosaurs; Palates with muscle attachments in left, lateral view of: B, Triceratops; C, Brachylophosaurus; D, Tyrannosaurus.
 

 

 Anatomical Abbreviations

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Fig. 7.Mandibular anatomy and m. pterygoideus ventralis attachment hypotheses in two derived non-avian dinosaurs. A, Brachylophosaurus: right, conservative, ventral mandibular attachment of m. pterygoideus ventralis; left, osteological correlates suggest a jugal attachment of the muscle; B, Soft-tissue anatomy of the medial portion of the mandible of the hadrosaur Edmontosaurus; C, Nanotyrannus: right, conservative, ventral mandibular attachment of m. pterygoideus ventralis; left, osteological correlates suggest a jugal attachment of the muscle; D, Soft-tissue anatomy of the medial portion of the mandible of the theropod Tyrannosaurus.  

Fig. 8. Jaw muscle anatomy in three dinosaurs in lateral view. Right, superficial muscles; left, deeper muscles. A, Edmontosaurus (CMN, 2289; modified from Rybczynski et al., 2008) with jugal removed; B, Diplodocus (CM 3452); C, Majungasaurus (FMNH PR2100).